1) Field of the Invention
The present invention relates to a technology for preventing position misalignment for each color occurring upon transferring a color toner image.
2) Description of the Related Art
An image forming apparatus generally employs a transfer method that forms a latent image for each color, produces a toner image using a developing unit, and then transfers the toner image to a recording medium using a transferring unit. There are three popular transfer methods as follow:    1) Method of employing an intermediate transfer element, forming a color image on the intermediate transfer element from photosensitive elements, and transferring the color image to a recording medium (hereinafter “intermediate transfer method”);    2) Method of conveying a recording medium on a transfer belt, and directly transferring toner images formed on the photosensitive elements to the recording medium in sequence (hereinafter “direct transfer method”); and    3) Method of combining the two above mentioned methods.
Currently, a high demand for printing color images calls for an increased need for the direct transfer method for balancing between printing cost and printing speed. However, a mechanism using the direct transfer method has a technological difficulty in position alignment between color images, which causes a faulty image to easily occur. Particularly, the position misalignment in each color at the time of image transfer is a technological problem to be solved.
In order to prevent the position misalignment, a couple of technologies were proposed. One of those technologies employs a color misalignment correcting unit that forms a plurality of mark patterns for each of colors arranged along a transfer belt, detects each of the marks by a sensor, and calculates a deviation amount of the mark from an ideal position to compensate for the deviation amount. The conventional technology is disclosed, for example, in Japanese Patent Application Laid Open No. 08-234531 and Patent Application Laid Open No. 2002-207338.
Japanese Patent Application Laid Open No. 62-226167 discloses a technology on a unit as follows. The unit detects marks previously formed on a transfer belt, calculates a moving speed of the transfer belt from a interval between the marks or detects a rotational speed at any part of a transfer belt drive system, feeds back the calculated moving speed or the detected rotational speed to a drive control circuit that controls to drive the transfer belt, and stabilizes the moving speed of the transfer belt (hereinafter, “belt speed correcting unit”).
However, as explained in the technologies, even when a deviation amount of the mark pattern is detected to correct it, or even when the moving speed of the transfer belt is detected to detect a deviation amount of the mark pattern and the deviation amount is corrected, the position misalignment or color misalignment is still a problem in the color printing. Inventors of the present invention examined the cause of the problem, and have found that the moving speed of the transfer belt when the deviation amount of the mark pattern is detected, is different from the moving speed of the transfer belt when a transfer medium is conveyed on the transfer belt and an image is actually printed on the transfer medium.